Unless otherwise indicated herein, the materials described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
Lithium-ion (Li-Ion) batteries are a popular choice for portable devices such as laptop computers, handheld computers, and similar devices because of their high energy density compared with other types batteries (e.g., Nickel-Cadmium batteries), their low self-discharge cycle, and absence of memory effects resulting in easier maintenance. Li-Ion batteries also have high charge-discharge efficiency (approximately 90%) and a relatively high nominal discharge voltage (e.g., 3.7 V). In applications where a supply voltage needs to be more than what can be obtained from a single battery cell, a battery pack including a plurality of serially-connected cells may be used.
The present disclosure appreciates that there are several limitations with conventional Li-Ion battery charge/discharge techniques. For example, in Li-Ion batteries the cell electrolyte breakdown is substantially near a full charge voltage (e.g., 4.2 V). Differences in internal impedance, uneven temperature distribution during usage, variations in the manufacturing processes, and aging may result in a difference in the characteristics of individual Li-Ion cells forming a battery during charge and discharge cycles. During the charging phase, these differences may result in some cells of the serially-connected battery pack becoming charged to the End of Charge (EoC) voltage faster than the remaining cells. Overcharge of these Li-Ion cells may lead to electrolyte fire, thermal runaway, or in a worst case scenario, an explosion. Similarly, during the discharge cycle, weaker cells may reach the End of Discharge (EoD) voltage earlier than the remaining cells. Over discharge of a cell (e.g., cell voltage<2.7 V) during the discharge phase may result in cell reversal and premature failure.
Alternative approaches to overcome the limitations of conventional Li-Ion battery charging/discharging techniques may include terminating the charging process as soon as any one cell voltage of the battery pack reaches the EoC voltage to prevent the weaker cells from being overcharged. This approach, in turn, may result in partial charging of the remaining cells in the battery pack lowering the overall battery capacity. To prevent cell reversal and consequent permanent cell damage, the battery may have to be disconnected from the load immediately resulting in the maximum battery capacity not being utilized. The battery capacity may thus be determined by the weakest cell of the battery pack.